1. Field of the Invention
The present invention relates to a signal level monitoring circuit for outputting voltage or current corresponding to an input signal level, particularly, a transmitting signal level which is output from a transmitting unit or a received signal level which is input to a receiving unit, in a multiplex communication apparatus used in microwave band (below, microwave multiplex communication apparatus).
2. Description of the Related Art
In general, a microwave multiplex communication apparatus includes a meter and the like, for displaying the transmitting signal level from the transmitting unit and the received signal level in the receiving unit, and has a structure which can monitor the transmitting signal level or the received signal level by converting a value displayed on the meter to a predetermined signal level by using a predetermined conversion table.
Accordingly, the transmitting unit or the receiving unit provided in the microwave multiplex communication apparatus comprises a signal level monitoring circuit consisting of a detector for detecting a transmitting signal or a received signal and an amplifier for amplifying an output signal level from the detector, in order to monitor the transmitting signal level or the received signal level; and a display unit, such as a meter, for displaying an output signal level from the signal level monitoring circuit.
Further, the signal level from the signal level monitoring circuit must be adjusted so that, when an input signal level RFin to the detector is a minimum input level RF.sub.min, the signal level from the monitoring circuit becomes a predetermined minimum monitor output level V.sub.min. Further, when the input signal level RFin to the detector is a standard input level RF.sub.typ the signal level from the monitoring circuit becomes a predetermined standard monitor output level V.sub.typ.
In order to provide a structure in which the minimum monitor output level Vmin and the standard monitor output level V.sub.typ of the monitor output signal are the predetermined values and the level of the monitor output signal can be adjusted so as to coincide with these output levels V.sub.min and V.sub.typ, a direct current amplifying circuit (below, a DC amplifier) which can determine an optional gain and an offset value, is used as the signal level monitoring circuit.